The present invention relates to radiation analyzers, and more particularly to apparatus for automatically controlling the gain of analyzer.
Conventional radiation analyzer instruments include a detector for providing an electrical pulse whenever a radiation particle is detected, and a pulse height analyzer for analyzing the resulting electrical pulses. The pulses provided by the detector have amplitudes which are directly proportional to the energies of the respectively detected particles. The analyzer determines the energy spectrum of the radiation to which the detector is responding by analyzing the distribution of amplitudes of the electrical pulses.
In order to correlate the pulse height distribution with an energy spectrum, the instrument must be calibrated so that a given pulse height can be identified as corresponding to a particle of known energy. Conventionally, calibration was accomplished by analyzing the radiation given off by a known source. Since the actual energy spectrum of that radiation source was known, the radiation analyzer could be calibrated by simply adjusting the system gain until the pulse height spectrum produced by the pulse height analyzer corresponded in scale with the energy spectrum of the source.
In these system, a radiation source (such as Americium 241) was used which had a pronounced peak in its radiation energy spectrum. This would, of course, cause the detector pulse amplitudes to tend to cluster around the pulse height corresponding to the energy of this peak, thereby producing a similar peak to occur in the pulse height spectrum accumulated by the analyzer.
The gain of the analyzer was automatically calibrated by counting the number of pulses occurring in the pulse height channels immediately above and below the pulse height at which it was desired to locate the centerline of the peak in the spectrum. If the system was properly calibrated, the radiation peak would exactly straddle that centerline, and the number of counts in the two windows would be approximately equal. If the gain of the system drifted such that the peak moved above or below that centerline, however, the number of counts occurring in the windows above and below the centerline would differ. This difference information was then used to readjust the system gain to place the peak back on the centerline.
In the system disclosed in the patent to Ladd et al., U.S. Pat. No. 3,270,205, the differences between the number of counts accumulated in the windows above and below the centerline was accumulated on a continuing basis, with the gain of the system being directly proportional to this accumulated count. In another system, disclosed in the patent to Laney, U.S. Pat. No. 3,609,362, the difference count was instead accumulated over fixed time intervals, and the gain was adjusted only if the difference count accumulated during those predetermined time intervals exceeded a preset limit.